1. Field of the Invention
The present invention relates to a printing press, and more particularly, to a plate loading method and device used for loading a plate in a predetermined position on a plate cylinder of the printing press.
2. Description of the Related Art
An offset press is typically constructed with three cylinders: a plate cylinder, a rubber blanket cylinder and an impression cylinder. In presses of this type, an inking system distributes ink and moisture to the plate cylinder which then transfers an ink image to the rubber blanket cylinder. The rubber blanket cylinder then transfers the image to a sheet of paper passing between the rubber blanket cylinder and the impression cylinder.
For efficient printing press operation, it is important to be able to transfer the printing plates on and off of the plate cylinder as quickly as possible. Typically, plates are prepared in the vicinity of the plate cylinder while the printing press is running. When the press stops, the used plate is removed from the plate cylinder and replaced with a new plate. It is important that the new plate be easily and accurately loaded onto the plate cylinder during this operation. One method used to load a plate onto a plate cylinder is disclosed in Japanese Laid-Open Patent No. 284252/1992 entitled "Device for Loading Plate on Plate Cylinder".
This method will be described with reference to FIGS. 6 and 7. In order to load a plate 53 onto the outer circumference of plate cylinder 51, as shown in FIG. 6, the plate cylinder 51 is equipped with a plate vice 52 for chucking the end portion of the plate 53. A housing 54, covering the plate cylinder 51 or the like, is opened at an entrance 55 facing the outer circumference of the plate cylinder 51. The entrance 55 is closed by a cover 56, while the plate cylinder 51 is rotating. When plate cylinder 51 is stopped for replacing the plate 53, cover 56 opens to expose plate 53. Cover 56 attaches to a plate receiver 57, a plate holder 58, a plate guide 59 and a plate tensioner 61.
According to the device described above, during the operation of the printing press, the entrance 55 is closed by the cover 56. Plate 62 is next loaded on the plate cylinder 51 at a position determined by the plate holder 58 and the plate receiver 57.
When the printing operation is complete, plate 53 is then replaced by another plate in the following manner. The cover 56 is first lifted, as shown in FIG. 7, to open the entrance 55. Then, the used plate mounted on the plate cylinder 51 is removed by opening the plate vice 52 and by rotating the plate cylinder 51. After the lower end portion of the plate 62, as indicated by a solid line, is removed from the plate holder 58 by the operator, and after it is inserted into the plate vice 52, as indicated by a double-dotted line, the plate 62 has its upper portion supported by the plate guide 59 and its lower end notches engaged with reference pins in the plate vice 52. When the upper end portion of plate 62 is inserted into the plate tensioner 61, plate 62 is warped into an arcuate shape and abuts against the combination of the plate tensioner 61, the plate guide 59 and the reference pins. This allows the lower end of plate 26 to be forcibly pushed onto the reference pins. When the plate vice 52 is then closed, the plate 62 has its lower end portion retained by the plate cylinder 51. Thus, according to this plate loading device, the plate can be efficiently replaced in a short time period.
However, when the plate 62 is to be loaded on the plate cylinder 51, it often becomes warped into an arcuate shape. In order to flatten plate 62, an elastic righting moment must be applied to the plate 62. This moment, however, causes the plate 62 to have its notches forcibly pushed onto the reference pins. As a result, after continuous loading and unloading of the plate 62 on and off the plate cylinder 51, the plate 62 may have its lower end gradually deformed or damaged. This deformation or damage in turn tends to deteriorate the printing accuracy of the printing operation.